Linear motion machines can be used for a number of different industrial operations in which work pieces are manipulated or measured. Linear motion machines can be configured to allow for motion along three linear axes. A linear motion machine can include a plurality of guideways that define each axis. Linear motion machines include, for example, bridge machines, gantry-type machines, and cantilevered machines. A bridge machine, for instance, generally includes a base, a bridge movably mounted to the base, and a carriage movably mounted to the bridge. An instrument can be movably mounted to the carriage and be used to measure or manipulate one or more work pieces placed and/or secured to the base. For example, a linear motion machine can be used to shape material. A cutting tool can be used to selectively remove material (e.g., metal or plastic) from a preformed body. Linear motion machines can also be used for assembling parts. For example, flat screen televisions and other electronic devices can be assembled by pick-and-place machines. Moreover, linear motion coordinate measuring machines can be used to measure the dimensions of various work pieces. Linear motion screen printing machines can be used for certain printing operations. Linear motion machines can also have high resolution measuring systems, electrical contact probes, motor drives, computer controlled drives, and computer acquisition and processing of data.
It is often desirable for linear motion machines to be accurate and precise. For example, it is often desirable for machine tools to be accurate and precise in order to repeatedly create articles to exact specifications. Pick-and-place machines with improved accuracy and precision can permit assembly of more compact and complex arrangements of various components. Coordinate measuring machines are used for dimensional inspection of work pieces such as machine parts and thus must be very accurate and precise in order to determine whether each work piece fits within a desired specification.
The accuracy of a linear motion machine can be limited by inaccuracies in the scales or other measuring systems and/or by faults in guideways that establish orthogonality of machine motions. For example, the bridge of a bridge machine may act as one of the guideways of a bridge machine. One approach to increasing accuracy is to improve the construction techniques and reduce tolerances of the system so that errors are reduced. Another approach is direct measurement of x, y and z errors at points throughout the linear motion machine working volume to permit correction of those errors. This approach, however, can be time consuming and data intensive. A third approach is to utilize measurements of errors in parametric form. That is, sets of error parameters are measured, for example, along three mutually orthogonal axes and stored for future use. The x, y and z errors at any point in the machine working volume are calculated from the parametric errors. For a coordinate measuring machine, the calculated errors are then subtracted from the scale readings to determine actual work piece coordinates. For a machine tool or pick-and-place machine, the parametric errors can be used to dynamically adjust the instrument relative to the base.
Significant errors can also be introduced because of environmental temperature changes. Environmental temperature causing significant expansion or contraction of different portions of linear motion machines can introduce noticeable errors into measurements and/or manipulations of work pieces. The expansion or contraction of a material due to a temperature change is described by a material's coefficient of thermal expansion (CTE). Numerical methods can be used to attempt to correct for environmental temperature changes based on the different CTEs of the different materials used in the linear motion machine. Numerical methods are not perfect; they can be limited due to non-instantaneous changes in the temperatures of the linear motion machine components. During a change in environmental temperature, each component can take time to cool or heat to the environmental temperature throughout its entire structure, and thus the components will continue to expand or contract until equilibrium is established. Therefore, selecting low CTE materials can minimize error associated with using the linear motion machine in a non-temperature controlled environment. Having a low CTE, however, is not the only criteria for selecting the material for the guideways of the linear motion machine. The modulus of elasticity (i.e., the stiffness) of a material is also a consideration. For example, a low modulus of elasticity can result in flexing and vibration of the cross-beam guideway of a bridge member of a bridge machine as the carrier moves along the cross-beam, which can introduce error.